The present invention relates to a process for preparing optically active phenylalanine.
A process for resolving DL-phenylalanine with an optically active mandelic acid has been already known (refer to "NIPPON KAGAKU ZASSHI" 92(11) 999-1002 (1971)). However, according to the present inventors' tracing of the publicly known process, the complex of phenylalanine and mandelic acid has a extremely low solubility in water as low as 2.6% by weight (calculated as phenylalanine) and therefore, the solubility ratio of the obtained complex of an optically active phenylalanine and the optically active mandelic acid to the mother liquor used in the resolution is extremely low. So, the known process is extremely disadvantageous in industrial scale.
As a result of the present inventors' studies for improving the disadvantage of the publicly known process, it has been found that complexes of DL-phenylalanine and an optically active mandelic acid is resolved in a solvent containing hydrochloric acid, a carboxylic acid, phosphoric acid or a salt thereof without decomposing the complexes and in addition, it has been found that the amount of the mother liquor used in the resolution is 1/2 to 1/6 of the amount of the mother liquor according the the known process of which such an acid compound or a salt thereof is not used. The present invention has been accomplished by the present inventors based on the knowledges mentioned above.